Pneumatically operated tools utilizing interior reciprocating striker mechanisms for punching holes through the ground are well known. In use such a tool is connected to a pneumatic pressure source and aimed in the desired direction. The pneumatic pressure causes a striker within the tool casing to reciprocate, thereby effectuating the self propulsion of the tool through the soil.
These tools are often employed in constructing pathways for installing utility piping an electrical cable in locations in which trenching would be difficult, such as underneath roadways. Frequently, the forward motion of the tool will be halted by obstructions in the soil. When such an obstruction is encountered it is desireable to reverse the direction of the tool so that the obstruction may be removed, or so that the tool may be diverted around the obstruction.
Numerous mechanisms and methods have been developed for selectively controlling the directional movement of these tools. Generally, tool directional reversal has been accomplished with rotatable valve members. Unfortunately, each such mechanism has exhibited certain undesirable characteristics in accomplishing tool reversal. Each such mechanism communicates the pneumatic pressure between an internal valve and the reciprocating striker at two different locations, one forward for forward tool direction, the other location being rearward for reversed tool direction. Tool propulsion is caused by the striker impacting with either the forward or rear end of the interior tool casing.
A number of prior art mechanisms for tool direction reversal propose to accomplish reversal with threads which shift the valve member and pneumatic supply conduit rearward. Such mechanisms operate by rotating the conduit and valve, a significant number of rotations. Use of such mechanisms in practice is extremely slow and difficult. Additionally, the difficulty of tool direction reversal is frequently exacerbated due to bends in the pneumatic supply conduit which inhibits conduit rotation and by partial cave-ins of the hole being formed. Unintended relative rotation of these screw type reversal mechanisms may also cause a loss of striker impact force during use.
Other prior reversal proposals effect tool direction reversal by rotating the valve member, supported by a valve guide. However, such mechanisms seem to be prone to inadvertent reversal. Accordingly, there have been proposals for improved reversing mechanisms which have sought to remedy inadvertent reversal by utilizing locking means to secure the valve assembly in either forward or reverse motion position, thereby preventing unwanted valve member rotation. One such mechanism utilizes a interengaging pin and slot arrangement. Directional reversal is accomplished by interrupting the pneumatic pressure supply and rotating the valve member and pins, out of their lock position before reconnecting the pneumatic pressure. Upon resupply, the pneumatic pressure longitudinally directs the valve assembly rearward, engaging the pins in rear pin holes, thereby locking the valve in reverse motion position. Tool operation then proceeds. Unfortunately, pneumatic pressure interruption consumes time and complicates tool operation.